This work describes an exploration of high-pressure acid gas chemical equilibria in CO2 rich fluids aimed at subsurface injection for acid gas reinjection, carbon sequestration, or enhanced oil recovery. To develop the capability to calculate both physical and chemical equilibria involving COS under high-pressure injectate conditions, we have measured the density changes associated with mixing COS (1) and CO2 (2) at temperatures between T = 322.91 and 393.25 K and up to p = 35 MPa. These density differences were used to calculate apparent molar volumes for COS, which were used to calibrate binary mixing parameters for high-accuracy reduced Helmholtz energy equations-of-state (gamma(v,12) = 1.0496 = gamma(T,12) = 0.94085). The new mixing parameters were shown to reproduce low-temperature p-x phase behavior measurements reported in the literature. The optimized mixing parameters were then applied in a Gibbs Energy Minimization (GEM) routine to find the enthalpy of formation for COS from literature equilibrium measurements at atmospheric conditions (Delta fH(298.15K,0.1 Mpa) (COS) = 141.744 +/- 0.803 kJ mol(-1)). The same GEM routine was used to explore the reaction of acid gases at applicable high-pressures. Results showed a decreased reaction of H2S + CO2 at low-temperature and low-pressure, that is, COS hydrolysis is less favorable at high-pressure.